1. Field of the Invention
The present invention relates to the packaging of semiconductor chips and particularly to that of electro-optical devices, such as a spatial light modulator (SLM).
2. Description of the Related Art
Packaging is a critical part of producing a high-performance electro-optical semiconductor device, such as a micromirror array or other spatial light modulator (SLM) for use in projection display systems. For example, in a micromirror, light is reflected by the mirrors, which, if in an ON state, form the projected image on a screen. To prevent stray light from reflecting from interconnect bond-pads, bond-wires, and other peripheral structures on the device, it is necessary to provide a light shield around the outer portion of the device. Such a shield typically has been provided by depositing an opaque metal on to the surface of the package""s cover glass and then opening an optically clear aperture in the center of the window to expose the mirrors to the light. This process has added cost to the package, introduced defects in the projected image from blemishes in the glass aperture, which is near the focal plane, and required a great deal of time and manufacturing capacity. As a result, the packaging of micromirror and other SLM chips for use in projection displays and other electro-optical applications has continued to present a cost barrier that contributes to higher priced projection display products.
FIG. 1 is a drawing of a typical micromirror package. In the past, this has often been a hermetic package, although lower-cost plastic packages are now becoming available, as referenced below. The package is comprised of a ceramic case 10 and an optical cover glass (lid) 11. The cover glass 11 has an aperture (clear opening) 110 and a light shield 120. The cover glass 11 is attached to the package case 10 by using hermetic welding or near-hermetic adhesive techniques. In the past, the purpose of the light shield 120 has been two-fold; (1) to prevent the interconnect bond pads and wires around the periphery of the micromirror from reflecting stray light and (2) to form a sharp border around the edge of the projected image. A getter material for absorbing moisture typically is attached to the bottom surface of the glass cover 11.
In these packages, the light shield 120 is applied to the glass cover 11 by vapor depositing a thin film metalization on one side (top or bottom) of the flat glass. The aperture 110 is then opened through the metalization by chemically etching the metalization in the center area of the cover glass 11, as shown. In a typical application, in addition to preventing stray light from getting into the projected image, the light shield 120 has been used to form a border around the edge of the projected image. In order to accomplish both of these functions, the shield must be in close proximity to the micromirror surface, such that the edge of the aperture is sharply focused. Since the aperture is deposited on the surface of the glass cover, the glass cover is also near the focal plane of the micromirror. As a result, even the smallest blemishes in the glass are in focus and therefore are visible in the projected image. This has required that high purity glass be used for the cover glass 11 and that these covers be handled with extreme care so as not to scratch the surface. Pure glass and special handling drive up the cost of the micromirror package.
There is a recognized need for a light shield that overcomes the limitations discussed above.
This invention discloses a drop-in aperture as an improvement for a high-performance, low-cost SLM package. The disclosed approach separates the metal aperture from the anti-reflective (AR) coated cover window and positions the aperture inside the package cavity in close proximity to the SLM""s surface. This allows the AR coated glass cover to be located away from the SLM and out of the focal plane, which considerably relaxes the blemish requirements for cover glass since the blemishes are no longer focused in the projected image. The relaxed blemish requirements enable the use of plastic covers.
In addition, this approach further balances the photopic appearance between the edges of the metal drop-in aperture and the on-chip micromirror light shield, so that the on-chip light shield is now used to precisely define the projected screen border. The makes the edge definition of the metal apertures less critical and loosens the alignment requirements of the drop-in aperture, further reducing the overall package cost.
Potential advantages of the disclosed invention are:
1. improved screen border definition,
2. easier assembly process, and
3. lower cost package.